Concrete, composed of cement, aggregate and water, is a well known building material having considerable compressive strength. There are multiplicities of application where low density concrete is a suitable, useful or desirable material since it has the advantage of light weight and favorable insulation properties.
In general there have been several methods to produce such low density concrete and lightweight aggregate. In one way, lightweight aggregate material, such as cinders available in ash heaps from coal-burning power plants, was used to produce such low density concrete products. However, a decade ago or more, when such cinders were no longer generally available, manufacturers substituted bloated slate, clays and shale, fly ash, pumice and the like which they produced in rotary kilns or sintering machines. While such kilned or sintered materials and methods using such heat-expanded materials are still currently in use, they are not very satisfactory or efficient as well as being increasingly very expensive due to material costs, fuel costs and labor costs. The expensive massive kilns or sintering equipment produce only relatively small amounts of product per working shift. Moreover, such heat-expanded aggregate making methods have not produced products with uniformly satisfactory properties. Besides requiring expensive and cumbersome machinery, heat-expansion processes create highly undesirable air pollution. Additionally, the specialized raw materials for producing such heat-expanded products are only available in certain limited geographic areas, often remote from the desired site for use.
Another manufactured lightweight aggregate is expanded slag. Hot dross is separated from the molton iron in steel production and is put in contact with water to cause bloating. Since the residue is a by-product, the aggregate is economical, but since it is dross it is neither uniform nor stable and therefore does not produce sufficiently uniform low density concrete.
Additionally, it has been suggested that low density concrete could be produced by making a "cellular concrete" by adding air-bubble containing foam to a concrete mix and trapping the air-bubbles therein. However, much of these bubbles are generally lost during the step in which the foamed composition is mixed with the concrete or during pouring of the concrete mix. The foamed compositions tend to break down or bubbles collapse and are lost during mechanical mixing of the compositions resulting in a large loss of air. Additionally, bubbles of the foamed composition tend to coalesce into each other and form relatively large and unstable air pockets, resulting in loss of cell integrity. Moreover, such cellular concretes have generally suffered from undesirable, unpredictable shrinkage and cracking during the curing or setting operation which tends to be erratic. All these factors tend to produce weakened cellular concrete. Also, such cellular concrete requires specialized on-the-job mixing apparatus, and the foam mix specifications must be tailorized for the necessary foam fluidity characteristics with increased water content needed to avoid undue loss of bubbles, rather than for the ultimate desired low-slump structural concrete mix specifications. Accordingly, such cellular concrete has found use primarily only in floor fills and roof deck applications, providing insulation and some modicum of fire protection, but due to the shrinkage and cracking or due to the need for specialized apparatus and the foam mix characteristics as described, conventional foamed concrete is generally unsuitable for use as a structural concrete.
One method of attempting to produce lightweight aggregate has been to provide a body of cured cellular concrete, breaking the body into fragments, coating the fragments with a thin layer of cement which is allowed to cure and incorporating the coated fragments in a cement matrix to form low density concrete. Such a method is disclosed for example in U.S. Pat. No. 4,351,670 issued Sept. 28, 1982 to Harold E. Grice. However, such products are not sufficiently stable and require a cumbersome process for preparation. In addition, such cellular concrete suffers from erratic curing or setting that results in setting-shrinkage or coalescing of cells and loss of cell integrity as discussed previously.
Moreover, the use of such cellular concrete to produce lightweight aggregate by heretofore employed methods has required the use of massive crushing equipment to transform the cellular concrete into suitable lightweight aggregate.
It is therefore an object of this invention to provide an easily and economically manufactured cellular concrete that can be used to produce concrete and/or insulation products with excellent and highly desirable structural, insulation, fireproofing and durability characteristics. It is also an object to provide cellular concrete of predictable density capable of being varied as may be desired over a wide density range and yet providing nearly perfectly controlled bulk densities, aggregate weights and strengths. It is a further object of this invention to provide cellular concrete that does not have erratic and abnormal setting and curing steps that result in undesirable shrinkage of the product and loss of cell integrity due to coalescing of the air-bubbles prior to setting of the concrete cells. Advantageously, this invention provides foamed concrete having a generally uniform tiny and symmetrical cell structure distinctly different from the structure of random larger and smaller cells and numerous interconnected and lopsided cells found in conventional foamed (cellular) concrete. A still further object of this invention is to provide cellular concrete with improved strength to weight ratio, decreased absorption and increased freeze-thaw durability. It is also an object of this invention to produce a cellular concrete in which setting-shrinkage is substantially eliminated and cell integrity is maintained.
A still further object of this invention is to provide lightweight aggregate material that is easily produced from cellular concrete.
An even still further object of this invention is to provide a lightweight aggregate for use in poured structural concretes, for steel fireproofing and for insulating concretes as well as for lightweight concrete blocks. Moreover, in view of the constantly increasing demand for manufactured lightweight aggregate, it is an object of this invention to provide an economically manufactured lightweight aggregate that can be produced in increased quantities to help meet the burgeoning demand.